Muse,
My take is that the PM came out this strongly against the idea of a Seraiki province to avoid the impression that the 'Sindh led' PPP was 'scheming' to break up Punjab.
The suggestion that the political parties should resolve the issue amongst themselves first is probably an indication that the PPP, overtly at least, wants the most popular party in Punjab (PML-N) to determine where it stands on the issue.
This is the rather unfortunate reality of our politics and politicians, that they would not be averse to playing the ethnic card to score political points.
For example, some PPP leaders tried raising a fuss over the GoPu's 'Sasti Roti' scheme, suggesting that somehow Punjab was discriminating against Sindh by not offering the scheme in Sindh! This occurred in the backdrop of the 'Long March to restore the judiciary', where the PPP was trying to undercut the support for the march and PML-N.
Its not the GoPu's job to run welfare schemes in Sindh, it is the GoS's job! Ajeeb loog hain hamaray siyastdan.
Similarly, I could see the PML-N leadership raising the ethnicity card if the senior PPP leadership was seen as championing the cause for more provinces. And Gillani is correct, such a mudslinging match would be a distraction from the other severe challenges we face currently, especially the PWaT.
While public support for Pakistan's War has remained steady, I would not put it past the PML-N to back out of its support, and the Taliban sympathizers in the media would have a field day with the PML-N opposition, and possibly shift public opinion again.
The creation of more provinces is something long overdue and necessary, but the potential costs of the PPP championing it are IMO not worth it.
Senior PML-N leaders (Jawed Hashmi etc., though not the Sharif brothers at this point) are reportedly backing the call for a new province, so there is still a chance that the call may gather momentum within the PML-N, and push the Sharif brothers into supporting it.
At the end of the day this will boil down to whether the PML-N sees a minimal loss of power with bifurcation of the province, which would involve an analysis of its party structure and reach in Southern Punjab, and whether a loyal (to Sharif brathran) and popular PML-N leadership in S Punjab exists, to maintain their overall vote bank.
There is also the possibility of the PML-N using the opportunity to call for divisions in other provinces as sell, to dilute the strength of the PPP, in which case it may very well be impossible to obtain political consensus on the creation of more provinces.
Personally I think that the Punjab has to be divided first, to negate the argument of 'Punjabi domination'. The arguments of the other provinces against division will be far weaker if the Punjab is already divided.